Hydrogen storage alloy electrode, battery including the same and method for producing the both

ABSTRACT

In the description, a hydrogen storage alloy electrode comprising a hydrogen storage alloy and a conductive metal and completely free of organic binder is disclosed, wherein at least two layers of an active material holding layer and a conductive metal layer essentially are integrated into an electrode sheet having a conductive network communicating throughout the electrode. The electrode can be used in a nickel-metal hydride storage battery, for example, particularly exhibits high efficiency charge/discharge characteristics while satisfying general characteristics as a battery, and has a relatively low cost and facilitates recycling.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a hydrogen storage alloy electrodewhich can electrochemically absorb therein (charge) and desorb therefrom(discharge) hydrogen and is applicable to storage batteries such asnickel-metal hydride storage battery.

2. Background Art

Recently, nickel-metal hydride batteries including a hydrogen storagealloy electrode as the negative electrode have rapidly become popular inour daily life because of their use as the secondary battery of compactportable appliances such as personal computer and cellular phone. Thenickel-metal hydride battery is characterized in that it has a 1.5- to2-fold higher capacity and is more pollution-free than theconventionally and widely used nickel-cadmium storage battery because ofno use of poisonous cadmium. More recently, in addition to compactportable appliances, application and inclusion of the nickel-metalhydride storage battery has been widened even to electric vehicle (EV)and hybrid electric vehicle (HEV) and further to electric instrumentsand emergency lights to which the nickel-cadmium storage battery hasconventionally been applied widely.

A conventional type of hydrogen storage alloy electrode used in thenickel-metal hydride storage battery has been produced mainly by a pastecoating technique. Such paste electrode is obtained by a method whichmixes a hydrogen storage alloy powder, organic binders such asstyrene-butadiene rubber (SBR) and/or carboxymethyl cellulose (CMC) anda conductive powder such as carbon, kneads the resultant mixture into apaste, then coats the paste on both surfaces of a low cost conductivecore member made of punched metal such as nickel-plated iron sheet andsubsequently dries, presses and cuts the conductive core member to makean electrode (see the publication of the U.S. Pat. No. 5,527,638, forexample).

The paste electrode produced by the paste coating technique isrelatively specific to mass production and there are many attempts torealize the mass production. However, the paste electrode still hasdrawbacks of unsatisfactory high rate charge/discharge performance andunfitness for use in making instantaneous charge and discharge at alarge current.

Another type of electrode is an electrode obtained by a method calledsintering. The Japanese-Laid Open Patent Publication No. Hei 3-294405,for example, discloses a method for continuous manufacturing of negativeelectrodes which supplies a hydrogen storage alloy fine powder to a wiremesh screen, then compresses the screen to form a deposit and sintersthe deposit, followed by quenching the sintered deposit in a gaseoushydrogen atmosphere. In this method, although improved high ratecharge/discharge performance can be expected as compared with the pastetype electrode, it also has drawbacks of much laborious process andliable reduction of alloy performance due to sintering, which havedisturbed wide use of sintered electrode.

Apart from the paste coating and sintering methods, a method has beenproposed in, for example, Japanese Laid-Open Patent Publication No. Sho62-216163 which obtains an electrode by molding a mixture of a hydrogenstorage alloy with a fluorocarbon resin binder into a sheet and thenmechanically pressing the sheet against a current collector. However,the electrode obtained by this method has a problem that it catches fireeasily, in addition to the conventional problem of unsatisfactory highrate charge/discharge performance.

The electrode obtained by any of the above-mentioned methods describedin the publication of the U.S. Pat. No. 5,527,638 and Japanese Laid-OpenPatent Publication No. Sho 62-216163 includes some form of organicbinder which is used as an essential constituent in the process ofelectrode production. The organic binder can be a factor for increasingelectrode resistance.

To the contrary, as the method which does not use the organic binder asthe factor for increasing electrode resistance, a method of producingelectrode by a dry press technique using a scaly copper or nickel powderhas been suggested in two Japanese Laid-Open Patent Publications No. Hei7-307154 and Hei 9-245797, for example. However, the electrode obtainedby this method is also disadvantageous in that due to insufficientcontact between the hydrogen storage alloy portion and conductive metalportion, an absolute content of the conductive material must beincreased in order to improve high rate charge/discharge performance.This type of electrode is considered to have another problem in terms ofcapacity density per electrode volume.

Despite many other proposed hydrogen storage alloy electrodes andmethods for producing the same, there has been an increasing demand fora simple and low cost method which can readily offer an electrode withparticularly excellent output characteristics.

Generally required conditions for the hydrogen storage alloy electrodefor use in the nickel-metal hydride battery and so on from the aspect ofthe intended use include: (i) excellent high rate charge/dischargeperformance; (ii) high durability and long service life; (iii) highenergy density; (iv) high conductivity; (v) high mechanical strength andruggedness; (vi) simple, low cost and easy production method of highutility; and (vii) easy-to-recycle.

It is true that hydrogen storage alloy electrodes obtained by thepreviously described prior art methods may satisfy the above conditionsconsiderably; however, there still exists a demand to further improvethe conditions (i), (vi) and (vii) in response to recent market requestsin particular.

Therefore, the primary object of the present invention is to provide anovel hydrogen storage alloy electrode which has excellent high ratecharge/discharge characteristics and facilitates recycling by using alow cost method of high utility.

SUMMARY OF THE INVENTION

The present invention is characterized by a hydrogen storage alloyelectrode comprising a hydrogen storage alloy and a conductive metal andcompletely free of organic binder, which is formed by integrating atleast two layers of an active material holding layer essentiallycomposed of a hydrogen storage alloy powder and a conductive metalpowder and a conductive metal layer essentially composed of theconductive metal into a sheet and is imparted with a conductive networkcommunicating throughout the electrode.

In the context of the present invention, the intended meaning of the“comprising” or “essentially composed of” is that other components mayalso be contained to an extent not to injure the effect of the presentinvention.

In a preferred mode of the present invention, the conductive metalcomprises Ni or Cu or an alloy containing Ni and Cu, and the activematerial holding layer essentially composed of the hydrogen storagealloy powder and conductive metal powder comprises 70 to 95 wt %hydrogen storage alloy powders and 30 to 5 wt % conductive metal powdersand the conductive metal layer comprises 95 wt % or more conductivemetal.

In another preferred mode of the present invention, a center of theelectrode is essentially composed of the conductive metal layer and bothends of the electrode are essentially composed of the active materialholding layer in a direction of electrode thickness.

Inversely, the center may be essentially composed of the active materialholding layer and both ends may be essentially composed of a porousconductive metal layer in a direction of electrode thickness.

In another preferred mode of the present invention, compositions of theconductive metal layer and the active material holding layer areinclined in continuity in a direction of electrode thickness.

Further, both ends of the electrode are preferably plated and haveconductive metal layers.

In a further preferred mode of the present invention, at leaset oneportion of the conductive metal layer is composed of a two-dimensionalor three-dimensional porous metal. The two-dimensional orthree-dimensional porous metal is desirably an embossed plate, punchedmetal sheet, expanded metal sheet, foamed metal sheet, lath metal sheetor metal fiber cloth.

Alternatively, it is also preferable that at leaset one portion of theconductive metal layer is composed of a metal foil.

One end or both ends of the electrode may be composed only of theconductive metal layer in a direction of electrode width.

In still another preferred mode of the present invention, the electrodehas a thickness of 0.5 mm or less and a porosity of 5 to 20%.

It is also preferable that the hydrogen storage alloy powder has anickel-rich surface by pretreated with hot alkali or acid.

It is also desirable that the hydrogen storage alloy powder is subjectedto mechanofusion or plating beforehand and has a surface having ametallic nickel layer.

The present invention also relates to a method for producing hydrogenstorage alloy electrode comprising a hydrogen storage alloy and aconductive metal and completely free of organic binder, comprising thesteps of:

(a) supplying a hydrogen storage alloy powder, a conductive metal powderand/or a porous conductive metal;

(b) laminating at least two layers of an active material holding layercomprising a mixture of the hydrogen storage alloy powder and/orconductive metal powder and a conductive metal layer essentiallycomposed of the conductive metal powder or porous conductive metal; and

(c) pressing a laminate produced by the previous step (b) to integratethe active material holding layer and conductive metal layer into asheet and to produce a conductive network communicating throughout theelectrode.

In the above-mentioned production method, step (b) and step (c) arepreferably performed concurrently.

It is particularly desirable that the step (b) and step (c) areperformed concurrently with a roll-press method using a pair of rollswhose surfaces have uneven parts.

It is also preferable that the mixture is heated in a non-oxidativeatmosphere for 10 minutes or less in a temperature range of not lessthan 500 C and not more than the lowest melting point of the meltingpoints of the metal elements constituting the electrode during or afterstep (c).

In that case, a desirable heating method is induction heating,electrical resistance heating, hot-press heating, or light beam heatingor heat ray irradiation heating.

The present invention also provides a battery including a hydrogenstorage alloy electrode comprising a hydrogen storage alloy and aconductive metal and completely free of organic binder, characterized inthat the hydrogen strange alloy electrode is formed by integrating atleast two layers of an active material holding layer essentiallycomposed of a hydrogen storage alloy powder and a conductive metalpowder and a conductive metal layer essentially composed of a conductivemetal into a sheet and has a conductive network communicating throughoutthe electrode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph illustrating a cross-section ofthe layer structure of the hydrogen storage alloy electrode produced inaccordance with one example of the present invention (×100).

FIG. 2 is a schematic view of an apparatus used for producing thehydrogen storage alloy electrode in accordance with one example of thepresent invention.

FIG. 3 is a schematic perspective view illustrating a hydrogen storagealloy electrode produced by the apparatus shown in FIG. 2 and used inone example of the present invention.

FIG. 4 is a schematic perspective view illustrating the surface shape ofrolls used in the present invention.

FIG. 5 is a schematic perspective view illustrating a hydrogen storagealloy electrode produced by the roll shown in FIG. 4.

FIG. 6 is a schematic perspective view illustrating the surface shape ofrolls used in the present invention.

FIG. 7 is a schematic perspective view illustrating a hydrogen storagealloy electrode produced by the roll shown in FIG. 6.

FIG. 8 is a schematic perspective view illustrating the surface shape ofrolls used in the present invention.

FIG. 9 is a schematic perspective view illustrating a hydrogen storagealloy electrode produced by the roll shown in FIG. 8.

FIG. 10 is a schematic view of a press-molding apparatus using aconventional roll.

FIG. 11 is a scanning electron micrograph illustrating a cross-sectionof the layer structure of the hydrogen storage alloy electrode producedin accordance with a comparative example (×100).

FIG. 12 is a scanning electron micrograph illustrating a cross-sectionof the layer structure of the hydrogen storage alloy electrode producedin accordance with another comparative example (×1000).

FIG. 13 is a graph illustrating the relationship betweencharge/discharge cycle and discharge capacity in electrodes A1, A2, Band C.

FIG. 14 is a schematic view of an apparatus used for producing thehydrogen storage alloy electrode in accordance with one example of thepresent invention.

FIG. 15 is a schematic view of another apparatus used for producing thehydrogen storage alloy electrode in accordance with one example of thepresent invention.

FIG. 16 is a schematic perspective view illustrating a hydrogen storagealloy electrode produced by the apparatus shown in FIG. 15 and used inone example of the present invention.

FIG. 17 is a graph illustrating the relationship betweencharge/discharge cycle and discharge capacity in sealed storagebatteries including electrode A2, C or F.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the present invention will be described referring toconcrete examples.

As described previously, the present invention relates to a hydrogenstorage alloy electrode comprising a hydrogen storage alloy and aconductive metal and completely free of organic binder, characterized inthat the electrode is formed by integrating at least two layers of anactive material holding layer essentially composed of a hydrogen storagealloy powder and a conductive metal powder and a conductive metal layeressentially composed of a conductive metal into a sheet and is impartedwith a conductive network communicating throughout the electrode andfurther to a method for producing the electrode. The present inventionalso relates to a battery obtained by using the hydrogen storage alloyelectrode and the method for producing the same.

More specifically, the electrode in accordance with the presentinvention is a hydrogen storage alloy electrode obtained bypressure-rolling at least two layers of an active material holding layerand a conductive metal layer to integrate the resultant laminate into asheet without use of the conventionally and widely used organic binder.

As stated before, there have been several attempts to remove the organicbinder used as a binding material in order to reduce resistancecomponents with electrode reaction or activity, However, prior methodshave failed to achieve satisfactory contact between the hydrogen storagealloy and conductive material. The present invention improves theabove-mentioned critical drawback of the conventional hydrogen storagealloy electrode and proposes an optimal electrode structure and a methodfor producing such hydrogen storage alloy electrode.

The conductive metal powder used in the present invention is preferablyNi or Cu or an alloy containing Ni and Cu. This is because both of Niand Cu have excellent electron conductivity, chemical stability andductility.

It is also desirable that the active material holding layer essentiallycomposed of the hydrogen storage alloy powder and conductive metalpowder comprises 70 to 95 wt % hydrogen storage alloy and 30 to 5 wt %conductive metal.

It is also desirable that the conductive metal layer comprisesconductive metal by 95 wt % or more of the resultant electrode. This isto improve the high rate charge/discharge performance of the resultantelectrode. As explained later, the conductive metal layer may becomposed of a conductive metal powder or of a porous conductive metal.The conductive metal layer may further comprise the above-mentionedhydrogen storage alloy in addition to the conductive metal.

It is desirable for the electrode that the center is essentiallycomposed of the conductive metal layer and both ends are essentiallycomposed of the active material holding layer in the direction ofelectrode thickness.

Inversely, the center may be essentially composed of the active materialholding layer and both ends essentially of the conductive metal layer.In this case, however, the conductive metal layer is desired to beporous in order not to inhibit the activity of the active materialholding layer in the center. The latter structure has effects ofimproving very large current discharge performance and elongatingservice life of the battery under severe use conditions.

In obtaining high output characteristics, the conductive metal layeracts as a key constituent to securing an electron-conductive networkthroughout the electrode and an essential constituent for securingelectrode flexibility.

From the aspect of further improving high rate charge/dischargeperformance of the electrode, at least one portion of the conductivemetal layer may be composed of a two-dimensional or three-dimensionalporous metal. As the two-dimensional or three-dimensional porous metal,an embossed plate, punched metal sheet, expanded metal sheet, metalsponge sheet or metal fiber cloth is desirable.

In this case, although Ni or Cu or an alloy containing Ni or Cu is adesirable material for constituting the two-dimensional orthree-dimensional porous metal, an Ni- or Cu-plated iron steel platewhich has low cost can also be used.

It is also preferable to incline the compositions of the conductivemetal layer and active material holding layer in continuity in thedirection of electrode thickness. Inclining the compositions not onlyimproves high rate charge/discharge performance but also has an effecton the cycle life characteristics and mechanical strength.

Particularly, continuous gradient of the compositions of the hydrogenstorage alloy powder and conductive metal powder is technicallyadvantageous in that required characteristics for practical battery,such as high rate discharge characteristic and cycle life characteristiccan be well-balanced.

Further in the present invention, the hydrogen storage alloy powder ispreferably pretreated with hot alkali, acid, mechanofusion or plating tohave a nickel-rich surface from the viewpoint that a conductive networkcommunicating through the electrode to be obtained can be surely formed.The electrode in accordance with the present invention yields a highdischarge rate and has satisfactory discharge performance. However, inorder to let the electrode exhibit its excellent performance as abattery, it is desirable to form a current collector in the electrode.In doing so, it is particularly effective to provide one end or bothends of the electrode with the conductive metal by plating in adirection of electrode width. Examples of the plated metal are Cu, Niand the like.

Direct electrical contact of the current collector with the conductivemetal at one or both ends enables to decrease internal resistance of theelectrode and improve high rate charge/discharge when it is assembledand operated as a battery. In addition, such conductive metal layer onthe surface of the electrode facilitates reduction of gaseous oxygen,which is generated around the end of charge, and enables acute and rapidcharge.

From the aspect of enabling rapid charge and discharge, it is preferablefor the electrode to have a thickness of 0.5 mm or less which is ratherthinner than usual. This range of thickness is desirable because theelectrode in accordance with the present invention can exert itscharacteristics with ease.

Furthermore, in order to secure high capacity density, stabilize cyclelife characteristics and enhance mechanical strength of the electrode,it is preferable to impart a relatively low porosity to the electrode byfilling the constituents at a relatively high density to make an overallporosity in a range of 5 to 20%. This is because when the porosity isless than 5%, there is a tendency that the probability of exposure of anelectrolyte with the electrode decreases and high rate dischargecharacteristic is impaired. To the contrary, when the porosity is morethan 20%, there is a tendency that cycle life characteristics areimpaired despite no particular problem in high rate dischargecharacteristic.

Next, the present invention also relates to a method for producing theabove-mentioned hydrogen storage alloy electrode.

In other words, the present invention relates to a method for producinga hydrogen storage alloy electrode essentially comprising a hydrogenstorage alloy and a conductive metal and completely free of organicbinder, comprising the steps of:

(a) supplying a hydrogen storage alloy powder, a conductive metal powderand/or a porous conductive metal;

(b) laminating at least two layers of an active material holding layercomprising a mixture of the hydrogen storage alloy powder and/or theconductive metal powder and a conductive metal layer essentiallycomposed of the conductive metal powder or the porous conductive metal;and

(c) pressing a laminate produced by the previous step (b) to integratethe active material holding layer and the conductive metal layer to makean electrode sheet and to impart to the electrode with a conductivenetwork communicating throughout the electrode.

The mixture of the hydrogen storage alloy powder and conductive metalpowder is molded into a high density sheet by pressure molding.

Exemplary applicable pressure molding methods include press-moldingusing a flat press machine, roll pressing using a pair of rolls andcalendaring.

In the production method in accordance with the present invention, it ispreferable to perform step (b) step (c) concurrently from the aspect ofefficient manufacturing process.

In doing so, if roll pressing using a pair of rolls is selected, it ispreferable to control supply of the hydrogen storage alloy powder andconductive metal powder as explained later referring to FIG. 2.

If an uneven part is formed on the surfaces of the pair of rolls, it ispossible to obtain a hydrogen storage alloy electrode having a surfaceshaped in correspondence with the shape of the uneven part.

FIG. 10 is a schematic view for illustrating a conventional rollingmethod using a pair of rolls.

The conventional roll rolling method supplies a mixture 3 of a hydrogenstorage alloy powder and a conductive metal powder between a pair ofrolls 5 by regulating supply of the mixture with a partitioning plate 1and molds the mixture into a sheet thereby to obtain a hydrogen storagealloy electrode 6.

By contrast, the method for producing hydrogen storage alloy electrodein accordance with the present invention supplies, to a pressure orcompression molding machine, the mixture of the hydrogen storage alloypowder and the conductive metal powder to be pressure molded incorrespondence with the intended structure of the active materialholding layer and conductive metal layer.

Here, the production method of the present invention will be describedby referring to the rolling method using a pair of rolls asrepresentative, with reference to FIG. 2. In this example, an electrodeis produced wherein the center comprises the conductive metal layercomposed of a conductive metal powder 4 and both ends comprise theactive material holding layer composed of the hydrogen storage alloypowder and the conductive metal powder in a direction of electrodethickness.

As shown in FIG. 2, two or more partitioning plates 1 are provided inaccordance with the layer structure of the resultant electrode, and amixture 3 of the hydrogen storage alloy powder and conductive metalpowder or the conductive metal powder 4 is introduced into a supplyingunit 2 between the partitioning plates 1 to supply it between the rolls5. This gives a hydrogen storage alloy electrode 6 comprising aconductive metal layer 7 and an active material holding layer 8 as shownin FIG. 3. FIG. 3 is a schematic view illustrating a hydrogen storagealloy electrode in accordance with one example of the present invention.The partitioning plate 1 may be shaped in the so-called hopper.

Then, the shape of rolls applicable to the method for producing hydrogenstorage alloy electrode in accordance with the present invention will bedescribed.

In the present invention, a roll having a smooth surface as illustratedin FIG. 2 may be used; however, the use of a variety of rolls havingsurfaces with different uneven parts is effective. This is because avariety of hydrogen storage alloy electrodes having surfaces withdifferent shapes in accordance with the shapes of the uneven parts canbe obtained. Such uneven shape may be exemplified as crepe weave andcaterpillar. Shapes as applied in usual embossing and gravure finish mayalso be used.

The rolls having such shape can prevent the powders to be pressurized orcompressed from slipping on the surface of the rolls and performsexcellent compression effect. Also, surface area of the electrode to beobtained can increase and improve reduction ability of gaseous oxygen,which is generated around the end of charge.

More specifically, the roll may have both or one of concave part andconvex part. In the use of a pair of rolls, the convex part may beformed on the surface of one roll and the concave part on the surface ofthe other roll. As such, the use of different combinations can offer avariety of hydrogen storage alloy electrode having different surfaceshapes.

As mentioned before, formation of uneven parts on the surface of thehydrogen storage alloy electrode has an effect of facilitating bendingof the resultant hydrogen storage alloy sheet and penetration of theelectrolyte into the hydrogen storage alloy electrode.

FIG. 4 to FIG. 9 show the shapes of uneven parts on the surfaces ofrolls and the shapes of the hydrogen storage alloy electrodes producedby using those rolls. FIG. 4, FIG. 6 and FIG. 8 show schematic views ofthe rolls applicable to the present invention and FIG. 5, FIG. 7 andFIG. 9 show schematic views of the hydrogen storage alloy electrodesproduced by using the respective rolls shown in FIG. 4, FIG. 6 and FIG.8.

A roll 5 shown in FIG. 4 is provided on its surface with two parallelconvex parts 5′ in a rolling direction. The hydrogen storage alloyelectrode 6 obtained by the roll 5 has on its both surfaces two concaveparts 11 in correspondence with the convex parts 5′ of the roll 5 asshown in FIG. 5.

The roll 5 shown in FIG. 6 is provided on its surface with parallelconvex parts 5′ in a rolling direction. The roll 5 can give a hydrogenstorage alloy electrode having concave parts 11 as shown in FIG. 7.

The roll 5 shown in FIG. 8 is provided on its surface with convex grids5′ and can give another hydrogen storage alloy electrode 6 whose surfacehas concave grids 11.

In each of FIGS. 5, 7 and 9, porous embossed core member made of Ni isemployed as the conductive metal layer.

Similarly, the use of a flat press machine can also offer the hydrogenstorage alloy electrode of the present invention having theabove-mentioned structure by modifying the supplying method of thehydrogen storage alloy powder and conductive metal powder asappropriate.

In a preferred mode of the method for producing hydrogen storage alloyelectrode in accordance with the present invention, the mixture of thehydrogen storage alloy powder and conductive metal powder is heated in anon-oxidative atmosphere for 10 minutes or less in a temperature rangeof not less than 500° C. and not more than the lowest melting point ofthe melting points of the metal elements constituting the alloy andconductive metal during or after the pressure molding step (c) to make asheet.

Such short-term heat treatment allows only the surface layer of thehydrogen storage alloy to react with the conductive metal layer to bebaked excluding reaction and baking of the interior of the hydrogenstorage alloy thereby imparting with a gradient function to theresultant electrode. As a result, an electrode can be obtained whereinthe hydrogen storage alloy and the conductive metal are firmly fixed oradhered to each other. It is also possible to further improve the highrate discharge characteristic, cycle life and mechanical strength of theelectrode.

It is desirable to perform the short-term heat treatment in anon-oxidative atmosphere such as inert gas atmosphere like argon gas orvacuum in order not to damage inherent functions of the hydrogen storagealloy and conductive metal due to their reaction in some form.

It is of particular importance to best prevent the hydrogen storagealloy from being oxidized by oxygen gas due to high temperature.

Desirable heating temperature used in the short-term heat treatment isnot lower than 500° C. and not more than the lowest melting point of themelting points of the metal elements constituting the electrode. At atemperature over the lowest melting point, baking of the metals having amelting temperature lower than that temperature proceeds in anaccelerated manner, which may in turn reduce the hydrogen absorbingcapacity of the hydrogen storage alloy excessively. Inversely, atemperature lower than 500° C. has no heating effect.

It is very important to shorten the duration of the short-term heattreatment as much as possible in order to facilitate control of baking,and a desirable duration is within 10 minutes, for example.

This time condition is very delicate; it has already been proven thatwhen the hydrogen storage alloy is baked in a conventional electricoven, reaction and baking of the hydrogen storage alloy and conductivemetal proceeds excessively and the hydrogen storage capacity of thehydrogen storage alloy is decreased excessively. Therefore, commonsensed short heating time is desirably within 10 minutes, more desirablyin seconds if possible.

It is preferable to perform the short-term heat treatment with inductionheating, excitation heating, or hot-press heating or light beam heatingor heat ray irradiation heating, for example.

In order to obtain an electrode with excellent performance, it isparticularly preferred to perform short-term heat treatment whilepressure molding a molding material. Treatment using a hot roll iseffective. Of the above-mentioned heating methods, excitation heating ismost effective. The reason is that since this method passes a currentthrough the electrode constituents, heating can be done effectivelyalong the distribution of current.

The use of induction heating is also effective because a ferromagneticpart of the hydrogen storage alloy having a nickel-rich layer isconsidered to be preferentially heated.

In the following, the present invention will be described morespecifically referring to concrete examples. However, the presentinvention is not limited only to those examples.

EXAMPLE 1

As the hydrogen storage alloy powder, an alloy of AB₅ type representedby the formula MmNi_(3.6)Mn_(0.4)Al_(0.3)Co_(0.7) was used. The powderwas obtained by mechanically grinding into fine particles an ingotobtained by thermally treating an alloy of the above compositionproduced by using a known high frequency induction melting and castingmethod. The powder has a particle size of about 30 μm. Some of the alloypowders were treated with alkali by immersion in an aqueous 30 wt % KOHsolution at 80° C. for one hour. It was confirmed that the surface layerof the hydrogen storage alloy powder treated with alkali was rich inmetallic nickel as compared with the surface layer of the hydrogenstorage alloy powder with no treatment with alkali.

Either hydrogen storage alloy powder was homogeneously mixed with an Nipowder as a conductive metal powder having a particle size of about 5 μmin a ratio of 10 wt % which gave a mixed powder. The mixed powder andthe Ni powder were molded into an electrode sheet using the apparatus asshown in FIG. 2, wherein the center comprised a conductive metal layercomposed of the Ni powder and both ends comprised an active materialholding layer composed of the hydrogen storage alloy powder and Nipowder in a direction of electrode thickness. Then, the sheet was cut toobtain a hydrogen storage alloy electrode. Here, the electrode obtainedby using non-treated hydrogen storage alloy powder was named “A1” andthe electrode obtained by using alkali-treated hydrogen storage alloypowder was named “A2”. In making electrodes A1 and A2, positioning ofthe partitioning plate 1 was controlled to make a thickness of about 60μm for the center in a thickness direction, that is, the conductivemetal layer. The electrode thickness was adjusted to about 300 μm.

Electrodes A1 and A2 are both hydrogen storage alloy electrodes formedby completely omitting organic binder wherein the active materialholding layer essentially composed of the hydrogen storage alloy and theconductive metal and the conductive metal layer essentially composed ofthe conductive metal were integrated into an electrode sheet impartedwith a conductive network communicating throughout the electrode.

Now, a scanning electron micrograph illustrating the cross-section ofthe layer structure of electrode A2 is shown in FIG. 1 (×100).

FIG. 1 is a 100 times magnified micrograph from which placement of theconductive metal layer in the center of the electrode can be clearlyseen. It can also be visually confirmed sufficiently that the conductivemetal layer and the active material holding layer are closely touchingand binding to each other and a conductive network is formed throughoutthe electrode. The upper and lower of the electrode of a three layerstructure in black indicates background.

COMPARATIVE EXAMPLE 1

A mixed powder was prepared by homogeneously mixing an identical alloypowder and an identical Ni powder to those of electrode A2 in Example 1in a ratio of 10 wt %. Then, the mixed powder was formed into anelectrode sheet comprising only the active material holding layer usingthe apparatus as shown in FIG. 10, which was named electrode “B”.

Electrode B as obtained had a thickness of about 300 μm. Anotherscanning electron micrograph illustrating the cross-section of the layerstructure of electrode B is shown in FIG. 11 (×100). FIG. 12 is a 1000times magnified micrograph. The upper and lower of the electrode inblack indicates background.

In electrode B, the Ni powder is dispersed around the hydrogen storagealloy to form an active material holding layer. However, there is noconductive metal layer which is an essential constituent of theelectrode in accordance with the present invention, rendering theelectrode mechanically fragile and collapsible.

From FIG. 11, it can be seen that pressure molding using the roll canoffer a molded sheet wherein all of the constituting powders are boundto each other with no use of binding material.

COMPARATIVE EXAMPLE 2

For comparison, a paste electrode “C” was produced by coating a mixedpaste of a hydrogen storage alloy, a carbon powder as a conductivematerial and SBR and CMC as organic binder onto a conventionally knownpunched metal, followed by drying and pressing the metal.

[Evaluation]

Electrodes A1 and A2 of the present invention and electrodes B and C ofcomparative examples were evaluated for their electrode performance.Performance of those electrodes as a hydrogen storage alloy electrodewas evaluated by assembling each of the electrodes into a half cell.

First, each of those electrode sheets were cut to a predetermined sizeto which an Ni ribbon lead was attached. As the counter electrode, anickel positive electrode having an excess capacity as compared to thatof the hydrogen storage alloy electrode was selected. Either hydrogenstorage alloy electrode and the nickel positive electrode as the counterelectrode were combined to form a sandwiched laminate by placing apolypropylene separator having hydrophilicity therebetween. Next, bothends of the laminate were sandwiched with plastic plates, which gave apressurized electric power generating element. The electrolyte used herewas an aqueous potassium hydroxide solution having a specific gravity of1.30. Each of the electric power generating elements as produced in theabove-mentioned manner underwent a charge/discharge test by regulatingthe capacity of the hydrogen storage alloy electrode.

At evaluation of electrodes A1, A2, B and C, the size or dimension wasadjusted almost equal and the thickness was adjusted so that the amountof the hydrogen storage alloy per unit area becomes almost equal. Atevaluation, the current rate was set on an assumption that the hydrogenstorage alloy yielded a discharge capacity of 300 mAh/g. The set currentrate was 1C for 300 mAh/g alloy.

First, at initial 5 charge/discharge cycles from 1st to 5th, a cycle ofa charge for 12 hours with a current of 0.1 C at 25° C. and a dischargewith a current of 0.2 C until the cell voltage drops to 0.8 V wasrepeated. The result showed that the discharge capacity at the 5th cyclewas around 290 mAh/g in the electrodes A, B and C, with no significantdifferences in discharge capacity and discharge voltage at a low ratedischarge as done in routine confirmation test of battery capacity.

Next, the electrodes were evaluated for their high rate dischargecharacteristic at the 6th and the subsequent cycles of thecharge/discharge test. More specifically, after a charge with a currentof 0.5 C at 25° C. for 2.5 hours, a discharge with a current of 1 C, 3 Cand 5 C was performed in each cycle until the cell voltage dropped to0.8 V. In order to evaluate high rate discharge characteristic at lowtemperature, a discharge with 1 C at 0° C. was also performed. Theresults are shown in Table 1. In Table 1, the discharge capacity at eachdischarge rate was expressed by capacity ratio (%) by defining the valueof discharge at 25° C. with 0.2 C as 100%.

TABLE 1 Results of discharge capacity test at each temperature and ateach discharge rate in electrodes A-C 25° C. 25° C. 25° C. 25° C. 0° C.0.2 C 1.0 C 3.0 C 5.0 C 1.0 C Electrode 294 mAh/g 91% 69% 42% 81% A1100% Electrode 292 mAh/g 93% 72% 50% 84% A2 100% Electrode 289 mAh/g 85%53% 21% 74% B 100% Electrode 290 mAh/g 90% 63% 34% 78% C 100%

As evident from Table 1, electrodes A1 and A2 in accordance with thepresent invention were found to exhibit much superior high ratedischarge performance to the electrode B and conventional electrode C ofcomparative examples in terms of high rate discharge characteristic with1 C, 3 C and 5 C and high rate discharge characteristic at lowtemperature of 0° C. with 1 C.

It was also found that as compared to the electrode A1 which includesnon-treated hydrogen storage alloy powder, the electrode A2 whichincludes alkali-treated hydrogen storage alloy powder has a betterdischarge characteristic.

A cycle life test was performed as a charge/discharge test at the 10thand subsequent cycles. A charge/discharge cycle was repeated under thetest conditions of a charge with a current of 0.5 C at 25° C. for 2.5hours and a discharge with a current of 0.5 C to make a terminal voltageof 0.8 V. The cycle at which the electrode capacity dropped to 70% orless of the initial discharge capacity due to the cycle life test wasassessed as the end of electrode life.

The results of cycle life test are shown in FIG. 13 which is a graphillustrating the relationship between charge/discharge cycle anddischarge capacity. As shown in FIG. 13, despite no use of organicbinder, electrodes A1 and A2 of the present invention achieved a cyclelife characteristic without any problem as compared to the conventionalelectrode C including an organic binder. Electrode B of comparativeexample ended its electrode life at a relatively earlier stage.

Those test results confirmed that electrodes A1 and A2 of the presentinvention achieved a drastic improvement of aimed high rate dischargecharacteristic and would be very effective if assembled into a sealedbattery without any problems in cycle life characteristic as concerned.

In the production method in accordance with the present invention, highperformance electrode A can be obtained by a material mixing step,pressure molding step, (heating step) and cutting step. To the contrary,the conventional paste coating method can only offer an electrode withinferior performance to the electrode of the present invention by thematerial preparing step (including paste), coating and molding step,drying step, pressurizing step and cutting step.

Therefore, it can be understood that the production method of thepresent invention enables production of an electrode with much simplersteps than the conventional method. A particular and great difference ofthe production method of the present invention from the conventionalmethod is treatment in a dry atmosphere. The production method of thepresent invention including the heating step will be explained in thefollowing examples.

EXAMPLES 2 to 6

Next, various electrodes were produced by varying the electrodestructure in a range of production method of the present invention onthe basis of electrode A2 produced in Example 1. In Examples 2-6, thesame hydrogen storage alloy powder as used in Example 1 was pretreatedto modify the surface of the hydrogen storage alloy with the sameelement as the conductive material.

More specifically, a fine Ni powder having a particle size of 0.03 μmwas mixed at 3 wt % of the hydrogen storage alloy powder and theresultant mixed powder was treated by mechanofusion in a gaseous argonatmosphere, using a mechanofusion device (Type AM-15F, manufactured byHOSOKAWA MICRON CORPORATION) (Example 2).

Separately, Ni was plated at 3 wt % onto the hydrogen storage alloypowder by using electroless Ni plating (Example 3).

These pretreatments gave hydrogen storage alloy powders whose surfacesare coated with Ni moderately. Using those pretreated hydrogen storagealloy powders, electrodes were produced following the procedure ofExample 1 (electrode A2). The electrode obtained by using themechanofusion-treated hydrogen storage alloy powder was named electrode“D” and the electrode obtained by using the Ni-plated hydrogen storagealloy powder was named electrode “E”.

Then, after producing a molded sheet in the same manner as in Example 1,the sheet was kept in a gaseous argon atmosphere. Subsequently, usingthe apparatus shown in FIG. 14, the sheet was subjected to resistanceheating by means of a seam welding roller 9 using a seam welding powersource 10 (Example 4). At that time, it was confirmed that the sheet washeated up to 950° C. at maximum by the resistance heating resulting fromcurrent supply between rollers 9. The sheet was heated for a very shorttime when it was in contact with the resistance heating rolls, followedby cooling in an environmental atmosphere in several seconds down to atemperature of 500° C. or less.

By the resistance heating, the sheet was baked in part and processedinto a sheet with a stronger mechanical strength. The electrode thusobtained was named electrode “F”.

Next, using the apparatus shown in FIG. 15, an electrode “G” in whichthe active material holding layers 3 were placed on both sides of theconductive metal layer 7′ (see FIG. 16) was produced in the same manneras in the case of the electrode “F” except that a porous embossed Nicore member 7′ formed with projections and depressions on both surfaceswas supplied into the center between a pair of rolls 5 in place of Nipowder as the material constituting the conductive metal layer and thatthe active material holding layers were formed on both sides of-theconductive metal layer by supplying the active material (Example 5).

Separately, an electrode was produced in the same manner as in Example 1except for the use of scaly Ni powder having a particle size of about 30μm (manufactured by FUKUDA METAL FOIL & POWDER CO., LTD.) in place of Nipowder having a particle size of about 5 μm as the conductive metalpowder (Example 6). The electrode obtained by this method was namedelectrode “H”.

Electrodes D-H in accordance with the present invention were evaluatedfor their electrode performance using the above-mentioned test method.Of the evaluation results, those of high rate discharge characteristicare shown in Table 2.

TABLE 2 Results of discharge capacity test at each temperature and ateach discharge rate in electrodes D-H 25° C. 25° C. 25° C. 25° C. 0° C.0.2 C 1.0 C 3.0 C 5.0 C 1.0 C Electrode 293 mAh/g 94% 74% 54% 86% D 100%Electrode 290 mAh/g 93% 72% 51% 83% E 100% Electrode 289 mAh/g 96% 79%64% 88% F 100% Electrode 289 mAh/g 95% 75% 52% 85% G 100% Electrode 289mAh/g 92% 73% 54% 85% H 100% Electrode 292 mAh/g 93% 72% 50% 84% A2 100%

As evident from Table 2, all of the electrodes D-H of the presentinvention have a superior high rate discharge characteristic toelectrode A2 of the present invention.

More specifically, improvements of pretreatment of the surface ofhydrogen storage alloy to modify the surface of hydrogen storage alloywith the same element as the conductive material, short-term heattreatment at the stage of forming a sheet of the hydrogen storage alloyand Ni powder, the use of scaly Ni powder in place of Ni powder, andarrangement of a conductive core in the center of the electrode areeffective. Of electrodes D-H, electrode F subjected to short-term heattreatment was particularly superior. Although not shown, the evaluationresults of cycle life characteristics of electrodes D-H were equal tothat of electrode A2.

EXAMPLES 7 to 18

In Examples 7-18, Ni and Cu were used as the conductive metal to examinetheir optical ratio to the hydrogen storage alloy.

First, the kind of used conductive material will be described. Two kindsof Ni were used: an Ni powder having a mean particle size of about 5 μmand a scaly Ni powder having a mean particle size of about 10 μm.Similarly, two kinds of Cu were used: a Cu powder having a mean particlesize of 45 μm or less and a scaly Cu powder having a mean particle sizeof 10 μm or less. As the hydrogen storage alloy, the same AB₂ type alloyas that shown in Example 1 represented by the formulaMmNi_(3.6)Mn_(0.4)Al_(0.3)Co_(0.7) was used.

The four kinds of conductive metal powders were mixed with the hydrogenstorage alloy powder to form 11 mixed powders by varying the ratio ofthe hydrogen storage alloy powder by 5 wt % in a range of 100 to 50 wt%. Then, 11 different electrodes were produced in the same manner aselectrode A2 of Example 1.

The electrodes thus obtained were similarly assembled into open halfcells and evaluated for their performance.

The evaluation results showed the following. As the key points inassessing characteristics of those hydrogen storage alloy electrodes,the following 3 points were taken into account particularly: 1) anelectrode having a high capacity density; 2) an electrode with excellenthigh rate discharge characteristic; and 3) an electrode exhibiting anexcellent cycle life characteristic.

The evaluation results showed that with increases of the ratio ofconductive metal powder, the volume capacity density at low ratedischarge of the electrode decreased. In order to secure a high capacitydensity, it is important to exclude the conductive metal powder.However, events observed in such electrodes with absent conductive metalpowder were: severe impairment of the high rate discharge performanceand extreme decrease of the cycle life characteristic.

Observation of those electrodes after evaluation revealed absence of theconductive metal powder, excess loss of the mechanical strength of theelectrode plate due to repeated charge/discharge cycles, falling off ofthe alloy due to pulverization of the alloy, and a decrease of electrodeconductivity. These events are estimated to have led to poor high ratedischarge performance and poor cycle life characteristic.

It was therefore confirmed that if the ratio of the conductive metalpower in the electrode is increased, the high rate dischargecharacteristic and cycle life characteristic would be improved almostcontinuously in correspondence with the ratio.

What performance is most important for the hydrogen storage alloyelectrode for use in nickel-metal hydride storage battery, high capacitydensity, high rate discharge characteristic or cycle lifecharacteristic, may slightly vary depending on the use condition of thebattery. However, in order to satisfy all of the above-mentionedimportant characteristics, it is preferable to form the active materialholding layer essentially composed of the hydrogen storage alloy andconductive metal in a ratio of 95 to 70 wt % hydrogen storage alloypowder and 5 to 30 wt % conductive metal powder.

It was found that ratios of the hydrogen storage alloy exceeding 95%pose problems of poor high rate discharge performance and poor cyclelife characteristic and that ratios of the hydrogen storage alloy lowerthan 50 wt % pose a problem of failing to offer a battery of highcapacity due to a decrease of capacity density.

With respect to the difference between Ni and Cu as conductive metalpowders, there was no significant difference in performance in theevaluation and the two elements were found effective as a conductivemetal. Observed minor differences may be due to the difference in powdershape. A tendency was observed that scaly powders exhibited superiorhigh rate discharge and cycle life characteristics to normal particulatepowders with relatively small amounts.

EXAMPLES 19, 20 AND COMPARATIVE EXAMPLE 3

Next, practical sealed batteries were produced using the hydrogenstorage alloy electrodes of the present invention.

Sealed batteries were produced using electrode A2 obtained in Example 1,electrode C obtained in Comparative Example 2 and electrode F obtainedin Example 4 and were compared for their characteristics.

The positive electrode was produced in the following manner. A pastetype nickel positive electrode called SME was produced by filling apaste of a mixture of a known particulate nickel hydroxide powder withcobalt hydroxide and zinc oxide into a porous nickel sponge having anetwork structure, followed by drying and pressing it. As the separator,a known non-woven polypropylene fabric whose surface is imparted withhydrophilicity was used. As the negative electrode, electrodes A2, C orF was used.

In order to enhance current collecting capacity as batteries, theelectrodes A2, C and F were modified to allow direct resistance weldingbetween the conductive metal layer and an electrode terminal byproviding a portion whose one end is composed only of the conductivemetal layer in the direction of electrode width.

As sealed batteries, the so-called SC size batteries having a diameterof 23 mm and a height of 43 mm were produced. An electric powergenerating element prepared by spirally winding three layers of thepositive electrode, separator and negative electrode was housed in abattery casing. For taking out the lead, the so-called ??tabless??current collecting structure used in usual high rate discharge wasadopted. Subsequently, an electrolyte dissolving 30 g/l lithiumhydroxide in potassium hydroxide having a specific gravity of 1.30 wasinjected into the battery casing and a metal jacket can and a sealingcap which is a lid with a safety valve were sealed with routine squeezesealing technique which gave a sealed nickel-metal hydride storagebattery. The battery is regulated for the capacity by the positiveelectrode to 3 Ah. 3 A is equal to 1 C.

Next, SC size sealed nickel-metal hydride storage batteries includingelectrode A2, C or F were produced each 5 pieces and received 5 cyclesof full charge and discharge with a relatively low current. In otherwords, a cycle of a charge at 25° C. for 6 hours with 0.2 C and adischarge with 0.2 C until the voltage dropped to 1.0 V was repeated.Charge and discharge at initial cycles confirmed that the batteries hadperformance as expected initially.

Subsequently, the batteries underwent a high rate discharge test at 25°C. with a current of 3.3 C and 10 C. The test condition includedrepeated cycles of a charge at 25° C. for 1.2 hours with 1 C and adischarge with a current of 10 A and 30 A until the terminal voltagedropped to 1.0 V.

Table 3 lists mean values of the intermediate discharge voltage anddischarge capacity ratio of the batteries including electrode A2, C orF. The discharge capacities at high rate discharge with 10 A and 30 Aare expressed as capacity ratios (%) by defining the discharge capacityat 25° C. with 0.2 C as 100%.

TABLE 3 Results of high rate discharge test in sealed batteries usingelectrodes A2, C and F Discharge at 10 A Discharge at 30 A Inter- Inter-mediate Capacity mediate Capacity voltage ratio voltage ratio Electrode1.18 V 98% 1.11 V 93% A2 Electrode 1.16 V 97% 1.08 V 90% C Electrode1.20 V 99% 1.15 V 97% F

From the above, it was clearly shown that the batteries using electrodesA2 and F in accordance with the present invention have markedly superiorhigh rate discharge performance to the battery using the conventionalelectrode C of comparative example.

Then, sealed batteries including electrode A2, C or F underwent thecycle life test. The test condition included repeated cycles of a chargeat 25° C. for 1.2 hours with 1 C and a discharge with a current of 1 Cuntil the terminal voltage dropped to 1.0 V. The cycle when theelectrode capacity dropped to 70% or less of the initial dischargecapacity by the cycle test was assessed as the end of life of thebattery.

The test results are shown in FIG. 17. As clearly seen from the figure,despite no use of organic binder, electrode A enabled cycle lifecharacteristics with no problem as compared with the conventionalelectrode C with organic binder. The test results confirmed thatelectrodes A2 and F in accordance with the present invention can improvedrastically the intended high rate discharge characteristic, and aremuch useful when assembled into sealed batteries with no problems incycle life characteristic which was a matter of concern.

A combination of various electrode constituting techniques as embodiedin the foregoing examples enables further development of the effect ofthe present invention. For example, a synergistic effect can be expectedfrom a combination of electrode F with electrode H.

As stated before, the present invention is not limited only to theforegoing examples and encompasses the range stated in the Disclosure ofInvention and the range of modifications and alterations made by theordinary skilled ones as appropriate.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to produce a hydrogenstorage alloy electrode particularly excellent in high ratecharge/discharge characteristics and facilitating recycling whilesatisfying performance of general requirement with a simple andrelatively cost effective method. The present invention can also providean epoch-making hydrogen storage alloy electrode by eliminating, becauseof no use of organic binder, removal of organic compounds contained inthe electrode which had long been problems at recovering and recyclingthe hydrogen storage alloy electrode.

The hydrogen storage alloy electrode, method for producing the same andthe battery using the electrode can be widely applied to the batteryfield.

We claim:
 1. A hydrogen storage alloy electrode comprising a hydrogenstorage alloy and a conductive metal and no organic binder, wherein saidhydrogen storage alloy electrode comprises at least two layers includingat least one layer each of: an active material holding layer essentiallycomposed of a hydrogen storage alloy powder and a conductive metalpowder; and a conductive metal layer essentially composed of saidconductive metal powder or a conductive metal porous material, saidactive material holding layer and said conductive metal layer beingintegrated to form said electrode having a conductive networkcommunicating throughout said electrode, wherein the electrode has majorsurfaces parallel to said layers and a thickness transverse to saidlayers, and wherein a center of the electrode in a thickness directionis composed of said conductive metal layer and the major surfaces of theelectrode are composed of said active material layers.
 2. The hydrogenstorage alloy electrode in accordance with claim 1, wherein theconductive metal of said active material holding layer and saidconductive metal layer comprises Ni or Cu or an alloy containing Ni andCu, said active material holding layer comprises 70 to 95 wt % hydrogenstorage alloy powders and 30 to 5 wt % conductive metal powders, andsaid conductive metal layer comprises 95 wt % or more conductive metal.3. The hydrogen storage alloy electrode in accordance with claim 1,wherein said conductive metal layer and said active material holdinglayer have a continuous composition gradient throughout the thickness ofthe electrode.
 4. The hydrogen storage alloy electrode in accordancewith claim 1, wherein at least one of said major surfaces of theelectrode is plated with a said conductive metal layer.
 5. The hydrogenstorage alloy electrode in accordance with claim 1, wherein at least aportion of said conductive metal layer is composed of a two-dimensionalor three-dimensional conductive metal porous material.
 6. The hydrogenstorage alloy electrode in accordance with claim 5, wherein saidtwo-dimensional or three-dimensional conductive metal porous material isan embossed plate, punched metal sheet, expanded metal sheet, metalsponge sheet, lath metal sheet or metal fiber cloth.
 7. The hydrogenstorage alloy electrode in accordance with claim 1, wherein saidelectrode has a thickness of 0.5 mm or less and a porosity of 5 to 20%.8. The hydrogen storage alloy electrode in accordance with claim 1,wherein said hydrogen storage alloy powder has a nickel-rich surface andis pretreated with hot alkali or acid.
 9. The hydrogen storage alloyelectrode in accordance with claim 1, wherein said hydrogen storagealloy powder has a surface having a metallic nickel layer and issubjected to mechanofusion or plating beforehand.
 10. A method forproducing a hydrogen storage alloy electrode comprising a hydrogenstorage alloy and a conductive metal and no organic binder, comprisingthe steps of: (a) supplying a hydrogen storage alloy powder, aconductive metal powder and/or a conductive metal porous material; (b)laminating at least one layer each of (i) an active material holdinglayer comprising a mixture of said hydrogen storage alloy powder andsaid conductive metal powder and (ii) a conductive metal layeressentially composed of said conductive metal powder or said conductivemetal porous material, and (c) pressing a laminate produced by step (b)to integrate said active material holding layer and said conductivemetal layer to make a sheet and to produce a conductive networkcommunicating throughout said electrodes wherein said step (b) and saidstep (c) are performed with a roll-press method using a pair of rollswhose surface has an uneven part.
 11. The method for producing ahydrogen storage alloy electrode in accordance with claim 10, whereinsaid step (b) and said step (c) are performed concurrently.
 12. Themethod for producing a hydrogen storage alloy electrode in accordancewith claim 10, wherein said mixture is heated in a non-oxidativeatmosphere for 10 minutes or less in a temperature range of not lessthan 500° C. and not more than the lowest melting point of the meltingpoints of the metal elements constituting said electrode during or aftersaid step (c).
 13. The method for producing a hydrogen storage alloyelectrode in accordance with claim 12, wherein the heating is performedby induction heating, excitation heating, hot-press heating, light beamheating or heat ray irradiation heating.